1. Field of the Invention
This invention relates to an optical type information recording-reproducing apparatus such as a magneto-optical disk apparatus, and particularly to a separation type optical head contained in such an apparatus.
2. Related Background Art
FIG. 1 of the accompanying drawings shows a perspective view of a prior art optical system for a disk-like recording medium. Information tracks T are formed on a disk D rotated by a driving device, not shown, and an objective lens 1 is disposed below the disk D. This objective lens 1 is movable in a direction X, i.e., the focusing direction, which is the direction of the optical axis thereof and a direction Y orthogonal to the direction X, i.e., the tracking direction, by an actuator, not shown. A laser beam emitted from a semiconductor laser source 2 irradiates the surface of the disk D via a collimator lens 3, a beam shaping prism 4, a first polarizing beam splitter 5, a mirror 6 and the objective lens 1. The laser beam L reflected and turned back by the disk D is divided into two laser beams by a second polarizing beam splitter 8 via the objective lens 1, the mirror 6, the polarizing beam splitter 5 and a condensing lens 7, and one of the two laser beams enters a servo sensor, not shown, and the other laser beam enters a third polarizing beam splitter 9 and is further divided into two laser beams thereby, and these two laser beams enter RF sensors, not shown. The difference between signals detected by these two RF sensors is found, whereby information recorded on the surface of the disk D is read. The optical system comprising these optical parts is provided in a carriage 10 and is adapted to be moved bodily with the carriage 10 in the seek direction S (the radial direction of the disk D).
FIG. 2 of the accompanying drawings is a perspective view of the optical system of a separation type optical head devised to shorten the seek time of the optical head. The optical system is separated into two by a carriage portion 13 in which a movable portion is constructed of only a necessary minimum portion to make the movable portion light in weight and a fixed head portion 14 fixed to the apparatus, and the polarizing beam splitter 5 and the mirror 6 which are in contact with each other in FIG. 1 are separated and disposed in face-to-face relationship with each other. The carriage portion 13 having the mirror 6 and the objective lens 1 is movable in the seek direction S by an actuator, not shown, and the objective lens 1 is supported for movement in the focusing direction and the tracking direction, and faces the disk D.
The laser beam L which has emerged from the polarizing beam splitter 5 travels parallel to the seek direction, enters the mirror 6 of the carriage portion 13 and irradiates the disk D. The laser beam L reflected by the disk D travels back along the optical path and returns to the fixed head portion 14.
In this separation type head, it is necessary that the laser beam L emerging from the fixed head portion 14 be made parallel to a guide rail for guiding the carriage portion 13 so that whether the carriage portion 13 is on the inner periphery or the outer periphery of the disk D, the position of the optical axis does not change. For this purpose, when mounting the fixed head portion 14 on a base bed, the fixed head portion is rotated in a direction a, b about a horizontal direction orthogonal to the seek direction S and a direction c, d about a vertical direction, to thereby adjust the direction of the laser beam L so as to be parallel to the seek direction S.
FIG. 3 of the accompanying drawings shows an adjusting mechanism for the direction a, b. Belleville springs 17 are inserted between the fixed head portion 14 and the mounting portion 16 of the base bed 15 thereof, and by these belleville springs 17 being adjusted, the fixed head portion 14 is inclined to thereby change the direction of the laser beam L.
However, in the example of the prior art described above, in the adjustment of the fixed head portion 14 in the direction a, b, the fixed head portion 14 is displaced relative to the horizontal direction to thereby accomplish the adjustment. This gives rise to a problem that the adjustment slips off by the temperature or heat cycle after the adjustment, or a variation with time or the like, and the optical axis moves to cause an increase in cross talk and a reduction in the reliability of an RF signal and further it becomes difficult for the servo to perform its function fully.